Recently it has been proposed to add dispatch capabilities to Code Division Multiple Access (CDMA) communications systems, such as a cdma2000 communication system. Unlike the interconnect services provided by today's cellular systems, dispatch services have been traditionally provided by two-way radio systems. Traditional dispatch services typically allow for instant access by a mobile station originating a call to a target mobile station. For example, a dispatch group call service enables a user to communicate with a group of people simultaneously and instantaneously, typically by depressing a Push-To-Talk (PTT) key. Using a cellular system, such a call could not occur instantaneously since either telephone numbers would need to be dialed for a three-way call or arrangements would need to be made to setup a conference call. A dispatch point-to-point call service enables a user to communicate with another user quickly and spontaneously, again typically by depressing a PTT key. This feature is ideal for two people who are working together but are unable to speak with one another directly such as two people working in concert but in different parts of a building. Where a wireless telephone call may be more appropriate for a conversation, short messages between two people as they work are better facilitated by the dispatch point-to-point call service.
Low delay is a critical factor in any dispatch call. For example, setup delay that is acceptable for a typical interconnect voice call can be unacceptable for dispatch services which rely on a very fast connection being made to the called party. Accordingly, dispatch services provide an instant access call setup. However, a problem in implementing a dispatch system in a cellular communication system is that the average time that it takes a user to navigate a phone book appearing on a display screen of a cellular phone, select an entry, and then set up a PTT phone call is anything but instantaneous.
In the proposals for implementation of dispatch in a CDMA system, it typically takes approximately 3-4 seconds to initiate a PTT phone call by a user of an originating cellular phone, that is, to depress a PTT key after getting to the cellular phone's phone book. Upon the user selecting an entry and depressing the PTT key, the cellular phone conveys a call origination message to the infrastructure identifying one or more cellular phones or a talkgroup associated with the selected entry. In response to receiving the call origination message, the infrastructure conveys a paging message to the one or more identified cellular phones or to one or more cellular phones associated with the identified talkgroup. In response to receiving the page, each called cellular phone wakes up and conveys a page response back to the infrastructure. A PTT phone call is then set up. This process of waking up a called cellular phone and establishing a PTT phone call may take another 3-4 seconds. In addition, the user of the originating cellular phone is not permitted to speak until receiving a Talk Permit Tone (TPT), which is not conveyed to the user until traffic channels are established between the infrastructure and the one or more called cellular phones. As a result, 9-10 seconds may expire between a time that the user of the originating cellular phone determines to initiate a PTT phone call and a time that the user is permitted to speak.
Therefore, a need exists for a method and apparatus that reduces a delay before conversation may be initiated in a PTT phone call in a cellular communication system.